Devices of the type to which the invention pertains are generally known, and they include for example facilites for the generation of phase shifted signals by operation of two sensors, whereby in addition, the two sensors are placed on an axis which is oriented at an actue angle in relation to the predominant extension of the passing markings. Increment pickup devices furish signals for obtaining digital machine control, particularly for controlling straight linear or rotating movements of the device such as a motor to be controlled so as to obtain for examp1e speed control in open or closed loop configuration. The actual speed is ascertained by the aforementioned increment sensors e.g. through pulse counting per unit time. The resulting signals update position counters the count state of which is used to control for example the drive motor and to correct its speed using drive pulses generated by currents that use the signals from the increment sensors.
Increment sensors are not only realized on the basis of optic/electronic principles, but sometimes different states of magnetism are used to meter the passage of physical increments. However the optical electronic appproach is preferred. The sensors and pickup devices are usually positioned in relation to a marking track so that they furnish two clock signals differing in phase by 90 degrees. In some instances a zero pulse is separately acquired. The sign of the phase difference of the signals as picked up is directly indicative of the direction of movement.
Turning now specifically to the preferred field of application it is known to provide for the open or closed loop speed and/or position control of a carriage in a matrix printer, carrying the print head. Optic electronic increment sensors are used in different configurations of scanning but always using transducers being comprised of phototransistors. Other motors in such a printer are controlled analogously. A typical device of the type to which reference is made presently is disclosed in German printed patent application No. 3,014,821 (see also U.S. Pat. No. 4,446,367).
It was found that phototransistors have certain drawbacks which cannot be neglected. The gain or amplification increases with increasing temperature but decreases with increasing frequency. Light emitting diodes are used as light sources in the pick up structure can be used, moreover, to compensate to some extent the temperature dependency of the phototransistors, simply because the illuminating output of a light emitting diode decreases with increasing temperature. Without active control, however, such compensation is to some extent a random phenomenon because the temperature rise in the phototransistor is not coupled, as far as circuit is concerned or otherwise, with the temperature rise of the light emitting diode. Also, the gain characteristics of a phototransistor does not coincide with the characteristic drop in luminous output of the diode. In such a case the diodes have to be specifically selected vis-a-vis the temperature characteristics of the phototransistor. In some cases the problem was solved in that phase shifted signals are used respectively as zero and 90 degrees signals. For purposes of controlling the luminous output of the diode a third transducer was used receiving radiation from the diode but not through the track. The output was then used in some form to obtain temperature compensation. However it was found in practice that temperature compensation in this manner is inadequate.